1. Field of the Invention
The present invention is in the field of electric motors, and, more particularly, is in the field of brushless DC electric motors for use in propelling vehicles such as watercraft.
2. Description of the Related Art
The need for the motor development described here comes from events that started a century ago when recreation size boats were propelled by electric motors. Eventually, such boats were converted to steam propulsion and then to propulsion by internal combustion engines. Except for small, slow-moving sightseeing boats and other limited horsepower boats, electric power is generally not used for recreational boats. Any trend towards the use of electric propulsion is generally stifled because of the inadequacies of the available batteries, which limits uses to low power, short duration applications.
Today, the economics of gas and diesel fuel and the atmospheric degradation caused by internal combustion engines have pushed for more extended use of electric propulsion as evidenced by hybrid automobiles and a few pure electric automobiles that have appeared over the last decade or so. The more extensive use of electric power for automobiles does not translate directly to corresponding use for recreational boats because of a combination of factors that are present in recreational boats that are not present, at least to the same extent, in automobiles. For example, boats are used in open water where one cannot easily obtain assistance. Unlike automobiles, boats are not readily adaptable to regeneration of electrical power, such as occurs when an automobile coasts and is slowed by braking. Recharging stations are not available on the open water. Notwithstanding the differences, it is desirable to produce an electric recreational boat to free the owners and their passengers from the bilious fumes, the explosive dangers, the noise, and of course the high operating costs associated with propulsion by internal combustion engines.
The use of electric propulsion in recreation boats is further complicated by the requirement that the electric propulsion system be safe beyond what is seemingly accepted for electrification of cars. Because of the inherently damp environment, the propulsion system should operate with a relatively low voltage. The batteries should be safe to handle and operate to substantially reduce or eliminate the possibility of an explosion or fire while in the open water or while docked proximate to other boats. Although no product can be totally safe, the reduction of the hundreds of volts required for a conventional high horsepower electric motor to a voltage that is generally considered to be safe is a basic safety requirement. As a result, the current state of the art of marine motors is less than 20 horsepower.
To provide an electric motor having the horsepower of a moderate-sized combustion engine for a recreational boat, such as for example, a 165 horsepower motor requires an electric motor having a rating of approximately 123 kilowatts. To obtain that power with a voltage no greater than approximately 50 volts, requires thousands of amperes of current. Such current is much greater than the currents in automotive use, which are typically in the low hundreds of amperes. Higher currents are generally beyond the capabilities of demountable contacts because of the contact resistance, for example. Furthermore, the resistive heating of the wiring caused by current flowing through the wiring would be excessive and would result in danger from overheating and breakdown of the wiring insulation.